The yellow material you are talking about,is a build up of food matter which accumulates in your throat/tonsil crevices.Whilst brushing your teeth,also brush your tongue.When you have done this,get some mouth wash and gargle,to release these particles. This will help to keep your tonsils cleaner and help stop particles of food accumulating on them.Brush after every meal but gargle as often as you can.

keep gums clean and healthy by mouth wash twice a day.
chew one cardamomum for every one 2 hrs.
gargle ur mouth with salt and lemon mixed water for 2 mts.
eat gooseberries fresh or dry for 1 week and see the results
above all keep ur mouth fresh by gargiling after every meal and if possible clean ur tongue.